ALBERT

Description: The eastern part of the oceanic Cocos Plate presents a heterogeneous crustal structure due to diverse origins and ages as well as plate-hot spot interaction. The complex structure of the oceanic plate directly influences the dynamics and geometry of the subduction zone along the Middle American Trench. In this work, an integrated interpretation of the slab geometry is presented based on three-dimensional density modelling of satellite-derived gravity data constrained by seismological information obtained by local networks. Results show the continuation of steep subduction from the Nicaraguan margin into northwestern Costa Rica followed by a shallower slab under the Central Cordillera toward the end of the Central American Volcanic Arc. To the southeast of the termination of the volcanic arc, the slab appears to steepen and continue as a coherent structure until reaching the landward projection of the Panama Fracture Zone. Overall a gradual change in the depth of intra-plate seismicity is observed reaching 220 km for the northwestern part and becoming shallower toward the southeast where it reaches a maximum depth of 70-75 km. The changes in the depth of the observed seismicity correlate with changes in the density structure of the subducting slab and may indicate that differences in the state of initial hydration of the oceanic lithosphere affect the depth reached by dehydration reactions in the subduction zone.

Description: The eastern part of the oceanic Cocos Plate presents a heterogeneous crustal structure due to diverse origins and ages as well as plate-hot spot interactions which originated the Cocos Ridge, a structure that converges with the Caribbean Plate in southeastern Costa Rica. The complex structure of the oceanic plate directly influences the dynamics and geometry of the subduction zone along the Middle American Trench. In this paper an integrated interpretation of the slab geometry in Costa Rica is presented based on 3-D density modeling of combined satellite and surface gravity data, constrained by available geophysical and geological data and seismological information obtained from local networks. The results show the continuation of steep subduction geometry from the Nicaraguan margin into northwestern Costa Rica, followed by a moderate dipping slab under the Central Cordillera toward the end of the Central American Volcanic Arc. Contrary to commonly assumed, to the southeast end of the volcanic arc, our preferred model shows a steep, coherent slab that extends up to the landward projection of the Panama Fracture Zone. Overall, a gradual change in the depth of the intraplate seismicity is observed, reaching 220 km in the northwestern part, and becoming progressively shallower toward the southeast, where it reaches a maximum depth of 75 km. The changes in the terminal depth of the observed seismicity correlate with the increased density in the modeled slab. The absence of intermediate depth (〉 75 km) intraplate seismicity in the southeastern section and the higher densities for the subducted slab in this area, support a model in which dehydration reactions in the subducted slab cease at a shallower depth, originating an anhydrous and thus aseismic slab.

Description: Within the project SFB574, an “amphibious” network of 15 ocean bottom seismometers and 27 land stations was operated from April to October 2008 along 350 km from the outer-rise to the magmatic arc. Additional readings from 11 permanent stations of the Chilean Seismological Service were included in the database improving onshore coverage. One of the main goals of the project is to gain a detailed image of the crustal and upper mantle structure and the seismogenic zone by analyzing earthquake distribution and combined passive and active source seismic tomographic images. To achieve precise earthquake locations and to serve as an initial model for local earthquake tomography, we derived a P- and S-wave minimum-1D model using a very high-quality subset of 340 events (GAP ! 180°, at least 10 P-wave and 5 S-wave arrivals) and velocity information from a wideangle profile shot in the area. Most of the ~1200 earthquakes recorded in our target area were originated within the subducting slab down to ~140 km depth, with a higher concentration beneath the main cordillera, at depths of 80-110 km. Fewer events were generated at the outer-rise, at depths of ~20-40 km, closely following the NE-SW trend of the oceanic plate faulting. The database was relocated using the minimum 1-D model and a subset of 400 events (GAP ! 180°, at least 8 P-wave arrivals) with ~7000 observations was selected to perform a P-wave tomography. Our results confirm the strong, lateral velocity gradient in the forearc seen in previous works along the margin, interpreted as the transition between a paleoaccretionary complex and the seaward edge of the Paleozoic continental framework. The downdip limit of the interplate seismicity previous to the great earthquake was aparently controlled by a low-velocity anomaly at ~40 km depth, shallower than the deeper extent estimated by geodetic modeling of the rutpture and from aftershocks relocation for the Maule earthquake. The interplate seismicity nucleated from ~40 up to ~20 km depth, and did not extend up to the 100°C isotherm. It was sparse except for a cluster of ~1200 km2 offshore and SW of Pichilemu town, within an area where a locking " 75 % before the great earthquake has been estimated. The deep outer-rise seismicity and the low velocities on top suggest considerable hydration of the downgoing plate.

Description: The Costa Rica Seismogenesis Project (CRISP) is designed to explore the processes involved in the nucleation of large interplate earthquakes in erosional subduction zones. On 16 June 2002 a magnitude Mw=6.4 earthquake and its aftershocks may have nucleated at the subduction thrust to be penetrated and sampled by CRISP, ~40 km west of Osa Peninsula. Global event locations present uncertainties too large to prove that the event actually occurred at a location and depth reachable by riser drilling. We have compiled a database including foreshocks, the main shock, and ~400 aftershocks, with phase arrival times from all the seismological networks that recorded the 2002 Osa sequence locally. This includes a temporal network of ocean-bottom hydrophones (OBH) that happened to be installed close to the area at the time of the earthquake. The coverage increase provided by the OBH network allow us to better constrain the event relocations, and to further analyze the seismicity in the vicinity of Osa for the six months during which they were deployed. We derived a minimum 1D model and used probabilistic earthquake relocation. Moreover, we undertook teleseismic waveform inversion to provide additional constraints for the centroid depth of the 2002 Osa earthquake. The latter, together with the maximum likelihood hypocenter, places the main shock origin at 5 to 10 km depth, ~30 km landward from the trench. Along the Costa Rican seismogenic zone, the 2002 Osa sequence is the most recent. It nucleated in the SE region of the forearc where this erosional margin is underthrust by a seamount covered ocean plate. A Mw=6.9 earthquake sequence occurred in 1999, co-located with a subducted ridge and associated seamounts. The Osa mainshock and first hours of aftershocks began in the CRISP area, ~30 km seaward of the 1999 sequence. In the following two weeks, subsequent aftershocks migrated into the 1999 aftershock area and also clustered in an area updip from it. The Osa updip seismicity apparently occurred where interplate temperatures are ~100°C or less. In this study, we present the relocation of the 2002 Osa earthquake sequence and background seismicity using different techniques and a moment tensor inversion for the mainshock, and discuss the corresponding uncertainties, in an effort to provide further evidence that the planned Phase B of CRISP will be successful in drilling the seismogenic coupling zone.

Description: Transition from subduction of normal to thickened oceanic crust occurs in the central portion of the Costa Rican margin, where large interplate earthquakes (M * 7) and abundant interseismic seismicity have been associated with subduction of bathymetric highs. We relocated *1,300 earthquakes recorded for 6 months by a combined on- and offshore seismological network using probabilistic earthquake relocation in a 3D P-wave velocity model. Most of the seismicity originated at the seismogenic zone of the plate boundary, appearing as an 18° dipping, planar cluster from 15 to 25–30 km depth, beneath the continental shelf. Several reverse focal mechanisms were resolved within the cluster. The upper limit of this interseismic interplate seismicity seems to be controlled primarily by the overlying-plate thickness and coherency, which in turn is governed by the erosional processes and fluid release and escape at temperatures lower than *100 to 120° C along the plate boundary. The downdip limit of the stick–slip behaviour collocates with relative low temperatures of *150 to 200° C, suggesting that it is controlled by serpentinization of the mantle wedge. The distribution of the interseismic interplate seismicity is locally modified by the presence of subducted seamounts at different depths. Unlike in northern Costa Rica, rupture of large earthquakes in the last two decades seems to coincide with the area defined by the interseismic interplate seismicity.